Electro Static Discharge Clamping Device

ABSTRACT

Electrostatic discharge clamp devices are described. In one embodiment, the semiconductor device includes a first transistor, the first transistor including a first source/drain and a second source/drain, the first source/drain coupled to a first potential node, the second source/drain coupled to a second potential node. The device further includes a OR logic block, a first input of the OR logic block coupled to the first potential node through a capacitor, the first input of the OR logic block being coupled to the second potential node through a resistor, and a second input of the OR logic block coupled to a substrate pickup node of the first transistor.

This is a divisional application of U.S. application Ser. No.12/329,415, which was filed on Dec. 5, 2008, and is incorporated hereinby reference.

TECHNICAL FIELD

The present invention relates generally to electro static discharge, andmore particularly to a semiconductor device for protecting againstelectro static discharge.

BACKGROUND

As electronic components are getting smaller along with the internalstructures in integrated circuits, it is increasingly easier to eithercompletely destroy or otherwise impair electronic components. Inparticular, many integrated circuits are highly susceptible to damagefrom the discharge of static electricity. Generally, electrostaticdischarge (ESD) is the transfer of an electrostatic charge betweenbodies at different electrostatic potentials or voltages, caused bydirect contact or induced by an electrostatic field. The discharge ofstatic electricity, or ESD, has become a critical problem for theelectronics industry.

Device failures resulting from ESD events are not always immediatelycatastrophic or apparent. Often, the device is only slightly weakenedbut is less able to withstand normal operating stresses. Such a weakeneddevice may result in reliability problems. Therefore, various ESDprotection circuits should be included in an integrated circuit toprotect its various components.

When a transistor is impacted by an ESD pulse, the extremely highvoltage of the ESD pulse can break down the transistor and canpotentially cause permanent damage. Consequently, the transistors of anintegrated circuit need to be protected from ESD pulses to prevent suchdamage.

Integrated circuits and the geometry of the transistors that make up theintegrated circuits continue to be reduced in size and the transistorsare arranged closer together. A transistor's physical size limits thevoltage that the transistor can withstand without being damaged. Thus,breakdown voltages of transistors are lowered and currents capable ofoverheating components are more frequently reached by the voltages andcurrents induced by an ESD event.

Thus, there is a need for small ESD protection devices that can berapidly triggered and conduct through the duration of the pulse, yet arerobust against spurious effects such as false triggering.

SUMMARY OF THE INVENTION

These and other problems are generally solved or circumvented, andtechnical advantages are generally achieved, by preferred embodiments ofthe present invention.

Embodiments of the invention include electrostatic discharge clamps. Inaccordance with a preferred embodiment of the present invention, asemiconductor device includes a first transistor, the first transistorcomprising a first source/drain and a second source/drain, the firstsource/drain coupled to a first potential node, the second source/draincoupled to a second potential node. The device further comprises an ORlogic block, a first input of the OR logic block coupled to the firstpotential node through a capacitor, the first input of the OR logicblock being coupled to the second potential node through a resistor, anda second input of the OR logic block coupled to a substrate pickup nodeof the first transistor.

The foregoing has outlined rather broadly the features of an embodimentof the present invention in order that the detailed description of theinvention that follows may be better understood. Additional features andadvantages of embodiments of the invention will be describedhereinafter, which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiments disclosed may be readily utilized as a basisfor modifying or designing other structures or processes for carryingout the same purposes of the present invention. It should also berealized by those skilled in the art that such equivalent constructionsdo not depart from the spirit and scope of the invention as set forth inthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawing, in which:

FIG. 1, which includes FIGS. 1 a-1 c, illustrates prior art electrostatic discharge clamp devices;

FIG. 2, which includes FIGS. 2 a and 2 b, illustrates an electro staticdischarge clamp device, in accordance with an embodiment of theinvention, wherein FIG. 2 a illustrates the circuit and FIG. 2 billustrates the operation of the MOS transistor of the ESD clamp;

FIG. 3, which includes FIGS. 3 a and 3 b, illustrates operation ofelectro static discharge clamp devices, in accordance with an embodimentof the invention;

FIG. 4 illustrates an electro static discharge clamp device, inaccordance with an embodiment of the invention;

FIG. 5 illustrates an electro static discharge clamp device, inaccordance with an embodiment of the invention;

FIG. 6 illustrates an electro static discharge clamp device, inaccordance with an embodiment of the invention;

FIG. 7 illustrates an electro static discharge clamp devices, inaccordance with an embodiment of the invention;

FIG. 8 illustrates an electro static discharge clamp devices, inaccordance with an embodiment of the invention;

FIG. 9 illustrates an electro static discharge clamp device, wherein theMOS transistor comprises a PMOS transistor, in accordance with anembodiment of the invention;

FIG. 10, which includes FIGS. 10 a-10 c, illustrates the layout of theMOS transistor of the electro static discharge clamp devices, inaccordance with an embodiment of the invention, wherein FIG. 10 aillustrates a top view and FIGS. 10 b and 10 c illustrate alternateembodiments of cross sectional view; and

FIG. 11 illustrates the layout of the MOS transistor of the electrostatic discharge clamp devices, in accordance with an embodiment of theinvention.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the embodiments andare not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

The present invention will be described with respect to preferredembodiments in a specific context, namely an electro static dischargeclamp. The invention may also be applied, however, to other types ofapplications and devices.

Gate-biased electro static discharge (ESD) power supply clamps are usedin the art for protection against ESD. Gate-biasing is generatedtypically from the output voltage of an RC-timer circuit which isamplified by one or more buffer stages. A standard RC-timed MOS powersupply clamp (“RCMOS”) is shown in FIG. 1 a. The clamp is triggered withthe rising edge of the ESD pulse and remains conducting as determined bythe RC-time constant. The RC time constant is determined by theresistance of the resistor 10 and the capacitance of the capacitor 20.The buffers 30 amplify the voltage at a node between the capacitor 20and the resistor 10 to transiently bias the gate of the MOS transistor100. However, after the transient (a time given by the time constant ofthe RC timer), the MOS transistor 100 of the RCMOS clamp is notconducting any more as the gate bias drops after the charging of thecapacitor 20 because the node voltage drops close to ground. Hence, ifthe time constant of the RC timer circuit is less than that of the ESDevent, the RCMOS clamp cannot provide a conductive path during the fullduration of the ESD event. As the pulse width of the ESD event and thetime constant of the RC circuit are independent, the RC-time constantneeds to be large enough to cover the duration of an ESD pulse. Further,the RC timer circuit has to be designed to accommodate the worst casescenario (largest pulse expected). Hence, in practice, the RCMOS clamprequires large area capacitors, and results in a deleterious increase instand-by leakage current.

To overcome the limitation of requiring large RC timer circuits, feedback circuits are added to the RCMOS clamps. For example, as illustratedin FIG. 1 b, additional, feedback loops are used to enhance thegate-bias effect and to increase the effective RC-time constant withoutrequiring more (IC-area consuming) capacitance. In FIG. 1 b, anadditional buffer 31 forms a feedback loop that sustains a longertransient at the gate of the MOS transistor 100. Thus, the gate biassignal at the MOS transistor 100 is sustained for a longer time than theoriginal RC-time constant of the RC circuit. However, RC-timed MOS powersupply clamps exhibit many problems including unintended triggeringand/or increased leakage currents during e.g. power ramp up ornoise/voltage spikes on the supply line. Particularly, RCMOS withfeedback loops have to be very carefully designed to avoid oscillation.Furthermore, all RC-timed MOS clamps are specifically sensitive totrailing electrical overstress (EOS), an unwanted phenomenon occurringin some ESD testing equipment, where parts of the remaining chargecannot be drained away due to the shut-off of the clamp afterRC-time-out.

Another approach in the art is the use of substrate pumping. Thesubstrate-pumped clamp, as illustrated in FIG. 2, features a pumptransistor that pumps current into the local substrate of the actual ESDshunt element (e.g. by a ring). The substrate pumping allows the MOStransistor 100 to uniformly conduct by a combination of MOS source todrain current arising from the extra gate biasing, enhanced MOS currentdue to the increased substrate potential and thus utilization of theMOS-body effect, and finally bipolar current due to the injectedsubstrate current acting as base current for the parasitic bipolar. Yet,as in the case of the RCMOS device discussed above, a RC-timer circuitis required to turn on the clamp (MOS transistor 100) for the completeduration of the ESD pulse. Another drawback is the large size requiredfor the pump transistor (within the buffers 30), which may typicallyreach the size of the clamp device itself.

Because of mainly MOS-based device operation, both clamping approachesdescribed above can be used without the need for silicide blocking.Silicide blocking is a process feature formed by blocking the formationof the silicide over the source/drain regions. The absence of silicideadds ballasting resistance in the source/drain regions. Silicideblocking would prevent failure of the device due to local heatingarising from non-uniform current distribution. With silicide blocking,the current would be forced through a larger region resulting in betterheat dissipation. Silicide blocking is not needed in the prior artdevices mentioned and is generally not preferred as it requires aseparate masking step resulting in an increase in production cost.

In various embodiments, the present invention overcomes theselimitations by providing an ESD clamp whose conduction is determined bythe ESD event (rather than an independent RC timer), that is robustagainst false triggering during power up, and/or against supply noise,and immune from trailing EOS. Further, the present invention in variousembodiments comprises a device that is fabricated in a small area due tothe low capacitive requirements, and consumes lower power (low leakage).

Embodiments of the invention use RC-triggering for generating a biasvoltage only for the initial turn of the ESD clamp comprising a MOStransistor. After the MOS transistor of the ESD clamp is turned on, thebias voltage for the remainder of the ESD pulse will be generated by theclamp itself. Further, the ESD clamp is “self-timed” to match theduration of the ESD pulse. In various embodiments, the clamping deviceis a MOS transistor that operates at the transition between MOS-mode andbipolar-mode. This inter mode operation of the MOS transistor results inan elevated substrate potential. The elevated substrate potential is fedback to a circuit from which the bias voltage is generated. However, theelevated substrate potential is only existent for the duration of theESD pulse resulting in an “auto-biased” device. Embodiments of thepresent invention disclosure include circuit implementations andadditional device design and layout techniques for realizing thisclamping technique for ESD protection.

In contrast to the above mentioned prior art, the auto-biasedself-timing (ABST) clamp does not inject a substrate current by apumping circuit, but picks up an inherent substrate potential. Unlikethe embodiment of FIG. 1 b, a circular feedback mechanism of the gatevoltage is not used. Rather, a moderate avalanche breakdown at the onsetof parasitic bipolar conduction makes the clamp advantageously immune tofalse triggering during normal circuit operating conditions. Embodimentsof the invention include omitting use of silicide blocking for the MOSclamp transistor, thus saving area and processing costs.

An embodiment of the invention will be described using FIG. 2.Simulations illustrating the operation of the ESD will be describedusing FIG. 3. Further embodiments of the ESD clamp will be describedusing FIGS. 4-9. Structural embodiment of the MOS transistor in the ESDclamp will be described using FIGS. 10 and 11.

FIG. 2, which includes FIGS. 2 a and 2 b, illustrates the ESD clampdevice in accordance with an embodiment of the invention.

FIG. 2 a illustrates an ESD clamp device in accordance with anembodiment of the invention. The ESD clamp device comprises a MOStransistor 100 coupled between power supply line VDD, and substratevoltage line VSS. A parasitic bipolar device 101 is also illustrated.The gate of the MOS transistor 100 is coupled to a RC timer circuitthrough an inverting buffer stage 30 (or a plurality of inverting bufferstages) and a NOR gate 40. In particular, a first plate of the capacitor20 is coupled to the power supply line VDD and a second plate of thecapacitor 20 is coupled to the substrate voltage line VSS through theresistor 10. The first input of the NOR gate 40 is coupled to the secondplate of the capacitor 20, while a second input of the NOR gate 40 iscoupled to the substrate of the MOS transistor 100. However, unlikeprior embodiments, the substrate of the MOS transistor 100 is coupled tothe second input of the NOR gate 40 through a pickup node P. Thecoupling of the substrate of the MOS transistor 100 to the second inputof the NOR gate 40 forms the auto biasing circuit.

The MOS transistor 100, in various embodiments, comprises a large width.In one embodiment, the gate length of the MOS transistor 100 may be at aminimum possible length, and, for example, about 50 nm in oneembodiment. The width of the MOS transistor 100 is at least 10 and about200 μm to about 400 μm in one embodiment. The local substrate of the MOStransistor 100 is coupled to the substrate voltage line VSS through asubstrate resistance (Rsub) depending on the design of the MOStransistor 100. In one embodiment, the local substrate is shielded fromthe substrate contact by counter doped regions. For optimal pickup ofthe local substrate potential, the substrate resistance Rsub is largerthan the pickup resistance between the junction of the MOS transistor100 (e.g., the source/substrate junction) generating the carriers andthe pickup node P of the MOS transistor 100. Hence, the pickup point Pfor picking the local substrate potential is closer to the junction ofthe MOS transistor 100 than the substrate pickup point for contactingthe substrate or body of the MOS transistor 100. Further, the pickupresistance is reduced as much as possible.

The operation of the ESD clamp device will be described using FIG. 2 b.The drain-to-source current (Ids) versus drain-to-source voltage (Vds)response of the MOS transistor (in linear scale) is illustrated in FIG.2 b. At low voltages the MOS transistor behaves like a conventional MOStransistor. Hence, when the gate bias is turned on due to the initialtransient of the RC circuit, a drain-to-source current flows. Hence, theNOR gate 40 outputs a low signal to the inverting buffer stage(s) 30 ifthe capacitor 20 is charging in response to an ESD pulse. The invertingbuffer stage(s) 30 outputs an amplified high signal on the gate of theMOS transistor 100 forming a conductive path between power supply line(Vdd), and substrate voltage line (Vss).

After this initial transient response of the ABST clamp which isdetermined by the RC timer circuit, the high electric field across thedrain and the substrate junction breaks down the junction due toavalanche breakdown. Consequently, this pulls up the local substratepotential of the MOS transistor 100. As the substrate node of the MOStransistor 100 is coupled to the second input of the NOR gate 40, thisresults in a transfer of the local substrate potential to the secondinput of the NOR gate 40. Again as the NOR gate 40 outputs a low signalto the inverting buffer stage(s) 30, and the inverting buffer stage(s)30 outputs an amplified high signal on the gate of the MOS transistor100. Thus, the substrate potential is amplified into a gate bias of theMOS transistor 100. The high gate bias preserves the inversion region ofthe MOS transistor 100 and maintains the conductive path between powersupply line (Vdd), and substrate voltage line (Vss). The conductionthrough the drain/substrate junction due to moderate avalanche breakdownstops when the drain voltage drops after the duration of the ESD pulse.Hence, the substrate voltage drops to a lower value closer to thesubstrate voltage line (Vss). As the substrate is coupled to the NORgate 40 through the pickup node P, the gate bias on the MOS transistor100 drops and the MOS transistor 100 stops conducting.

Thus, the auto-biased self-timed ESD clamp uses RC-triggering only forproviding a bias signal for initial turn like an RCMOS. The gate biasvoltage for the main portion of the ESD pulse is generated by the clampitself. After RC-time out, the clamping device is operated at thetransition between MOS-mode and bipolar-mode which results in anelevated substrate potential. The clamp remains in conducting mode in aself-sustaining way through the duration of the ESD pulse, after whichthe clamp turns off.

In contrast to prior art, the ABST clamp does not pump the substrate anddoes not feed back its own gate bias. Rather, the ABST clamp generatesits own gate biasing by feeding back its own substrate potential. As thedevice is operated by moderate avalanche breakdown and at moderatelyhigher clamping voltages at the transition region from MOS conduction toparasitic bipolar conduction, the ABST clamp is immune to falsetriggering during normal circuit operating conditions, which aresubstantially lower than supply voltage Vdd. In various embodiments,silicided source/drain regions are formed on the MOS transistor 100without the need for blocking the formation of silicide regions. Thissaves an extra mask step along with the related processing saving areaand processing costs.

FIG. 3, which includes FIGS. 3 a and 3 b, illustrates the detailedoperation of an auto biased self timing ESD clamp, in accordance withembodiments of the invention.

Circuit simulations of a 2 kV human body model (HBM) ESD dischargeillustrate the operation and advantage of the Auto-Biased Self-Timed(ABST) MOS Clamp, in accordance to embodiments of the invention. InFIGS. 3 a and 3 b, the ABST clamp is compared against a RCMOS clamp asdescribed in FIG. 1 a.

The ABST clamp demonstrates an improved gate bias voltage V(Gate) overthe RCMOS clamp during the entire duration of the ESD pulse and resultsin a better clamping as seen by the reduced drain voltage (vdd). Thesimulation is performed using a RC timer circuit with a time constant of20 ns which is much smaller than the time constant of a human body model(HBM) ESD pulse, which is typically about 150 ns. For the simulationillustrated in FIG. 3, the gate length of the MOS transistor 100 isabout 230 nm, and the width is about 1000 μm.

The drawback of the reference design is clearly visible in FIG. 3 a. Asillustrated in FIG. 3 a, after the time constant of the RC timercircuit, the gate voltage V(Gate) of the MOS transistor 100 drops. Theincreasing drain voltage V(vdd) results in the break down of thedrain-substrate junction due to a hard avalanche breakdown resulting ina breakdown current I(Ddb). In contrast, the ABST clamp device shows amuch smaller drop in gate voltage V(Gate) resulting in continued flow ofsource/drain current (Id). Hence, an effective clamping of the drainvoltage is achieved.

FIG. 3 b illustrates the robustness of the ABST design by comparing thereference RCMOS clamp and the ABST clamp using two different RC timercircuits. The time constant of the first RC timer circuit is about 10ns, whereas the time constant of the second RC timer circuit is about 20ns. This is visible in the curves illustrating the gate voltage of theRCMOS clamp (labeled “RC time out”). The actual duration of the gatevoltage is about 35 ns and 55 ns for the first and the second RC timercircuits respectively for reference RCMOS. The gate voltage of the ABSTclamp is much higher, and almost indistinguishable for both timeconstants. Hence, as expected, the drain voltage clamping is alsosimilar for the ABST clamps using the different RC timer circuits.Unlike the RCMOS clamps, the voltage clamping of the ABST clamp isindependent of the RC time constant of the RC circuits illustrating therobust applicability and proof of the concept of auto biasing inaccordance with an embodiment of the invention.

FIG. 4 illustrates an embodiment of the ESD clamp device in accordancewith an embodiment of the invention. In this embodiment, the NOR gate 40and the inverting buffers 30 are replaced by an OR gate 42. Hence, theoperation of this device is similar to that described above.

FIG. 5 illustrates an embodiment of a circuit implementation of theinvention using ABST techniques discussed in FIG. 2. The ABST clampcomprises a NOR gate 40 comprising a first NMOS transistor N1, a secondNMOS transistor N2, a first PMOS transistor P1, and a second PMOStransistor P2. A third PMOS transistor P3 and a third NMOS transistor N3form an inverting buffer stage. The RC timer circuit comprises acapacitor 20 and a resistor 10, the RC timer coupled to the NOR gate 40and the power supply lines as described above with respect to FIG. 2.The MOS transistor 100 comprises a fourth NMOS transistor N4. The fourthNMOS transistor N4, in various embodiments, comprises a large widthtransistor. In various embodiments, the gate length of the fourth NMOStransistor N4 is a minimum length transistor. In one embodiment, thegate length of the fourth NMOS transistor N4 is about 50 nm to about 100nm. The width of the fourth NMOS transistor N4 is at least 10 μm in oneembodiment.

FIG. 6 illustrates a simplified embodiment of a circuit implementationof the invention using ABST techniques discussed in FIG. 2.

Unlike the embodiment of FIG. 2 a, in this embodiment, the NOR gate ofFIG. 2 a is replaced by an inverting buffer stage. Hence, the circuitcomprises a first inverting buffer stage comprising a first PMOStransistor P1 and a first NMOS transistor N1, and a second invertingbuffer stage comprising a second PMOS transistor P2 and a second NMOStransistor N2 (MOS transistor 100). In this embodiment, some of theinitial trigger signal of the RC circuit is also used for generating asubstrate bias. After timeout of the RC circuit, the circuit draws asignal from the local substrate pickup and generates the gate bias forthe MOS transistor 100.

FIG. 7 illustrates an embodiment of a circuit implementation of theinvention using ABST techniques discussed in FIG. 2. This embodiment issimilar to that shown in FIG. 5, but includes additional invertingbuffer stages.

As described with respect to FIG. 5, the NOR gate comprises a first NMOStransistor N1, a second NMOS transistor N2, a first PMOS transistor P1,and a second PMOS transistor P2. A first inverting stage comprising athird PMOS transistor P3 and a third NMOS transistor N3 is coupled tothe NOR gate.

Unlike FIG. 5, two additional inverting buffer stages are coupled to thegate of the MOS transistor 100. Hence, the gate bias on the MOStransistor 100 is amplified more than the embodiment of the FIG. 5. Asecond inverting stage comprising a fourth PMOS transistor P4 and afourth NMOS transistor N4 is coupled to the first inverting stage. Athird inverting stage comprising a fifth PMOS transistor P5 and a fifthNMOS transistor N5 is coupled to the second inverting stage. The thirdinverting stage is coupled to the MOS transistor 100. Although in thisembodiment two additional inverting buffer stages are added, in otherembodiments, more number of inverting buffer stages may be added. Invarious embodiments, the inverting buffer stages are added in incrementsof two till the required gate signal amplification is achieved.

FIG. 8 illustrates an embodiment of a circuit implementation of theinvention using ABST techniques discussed in FIG. 2. In variousembodiments, the capacitance required for the triggering the ABST clampis small due to the dependence on the RC circuit only for the initialtriggering. This is in contrast to the RCMOS devices that require largecapacitors. Hence, in some embodiments, the intrinsic capacitance of theMOS transistor 100 is used as the initial triggering capacitor. Duringnormal operating conditions, the third NMOS transistor N3 is “on” actingas a resistor, similar to the resistor 10 of the RC timer circuit.

Referring to FIG. 8, the drain-gate capacitance (Cdg) of the MOStransistor 100 is used similar to the RC timer circuit. The drain-gatecapacitance (Cdg) arises primarily due to the overlap of the drainextension regions under the gate electrode of the MOS transistor 100. Invarious embodiments, the MOS transistor 100 can be designed to vary thisintrinsic capacitance also called Miller capacitance. Consequently usingthe embodiment, the external RC circuit is eliminated.

FIG. 9 illustrates an embodiment of the ABST clamp device using a PMOStransistor as the clamping transistor. Although the embodiments of FIGS.5-8 are described using a NMOS transistor as the MOS clamp transistor,other embodiments may use PMOS transistors.

Referring to FIG. 9, the placement of the resistor 10 and the capacitor20 is exchanged relative to FIG. 2 a. Further, the NOR gate 40 in FIG. 2a is replaced by a NAND gate 43. The substrate resistance Rsub to thesubstrate voltage line VSS is replaced with the nwell resistance Rnwellto the power supply line VDD. Thus, the NAND gate 43 is coupled to theinverting buffers 30 forming an AND logic block.

In particular, a first plate of the capacitor 20 is coupled to the powersupply line VDD through a resistor 10 and a second plate of thecapacitor 20 is coupled to the substrate voltage line VSS. The firstinput of the NAND gate 43 is coupled to the first plate of the capacitor20, while a second input of the NAND gate 43 is coupled to the localsubstrate (nwell) of the MOS transistor 100 through a pickup node P. Thecoupling of the local nwell potential of the MOS transistor 100 to thesecond input of the NAND gate 43 forms the auto biasing circuit.

FIGS. 10 and 11 describe the layout of the MOS transistor used invarious embodiments described in FIGS. 2-9. FIG. 10, which includes 10a-10 c, illustrates the layout of MOS transistor in accordance with anembodiment of the invention.

FIG. 10 a illustrates a top view of an embodiment of the MOS transistor100 as described in, for example, FIG. 2. FIGS. 10 b and 10 c illustratecross sectional views of FIG. 10 a according to alternate embodiments ofthe invention.

Referring to FIG. 10 a, the MOS transistor 100 (a NMOS transistor as anexample) comprises a first ring 120 (for example, a p type region)coupled to a standard substrate potential node. A second ring 130comprising an n type region is disposed around the first ring 120. Invarious embodiments, the second ring 130 is floating or coupled to areference potential. A gate 150 (for example, U shaped gate) is disposedcentrally forming the MOS transistor 100. The various regions arecontacted using contacts 160. Further, source (S), and drain (D) regionsof the MOS transistor 100 are illustrated in the top view. In variousembodiments, the number or transistor fingers can vary, and in oneembodiment determined by the targeted ESD hardness of the clamp device.

The MOS transistor 100 additionally comprises a pickup region 140 (athird ring) disposed between the gate 150 and the second ring 130(n-well). In various embodiments, the pickup region 140 comprises anysuitable shape. The pickup region 140 (e.g. a p type doped region in theshape of a ring) is coupled to the substrate and comprises contacts thatform the substrate pickup nodes. In various embodiments, the pickupregion 140 comprises a p+ region for efficient pick up of the substratepotential without resistive losses. The pick up region 140 is shieldedfrom the first ring 120 by the second ring 130 because the second ringis floating or coupled to a fixed potential node (for example, drainvoltage Vdd).

The cross section views of FIG. 10 a are illustrated in FIGS. 10 b and10 c which illustrate alternate embodiments. FIG. 10 b illustrates adual well process, whereas FIG. 10 c illustrates a triple well process.Embodiments of the invention described above may include any of thecross sections illustrated in FIGS. 10 b and 10 c.

The MOS transistor 100 comprises source regions 170 and drain regions180 separated by channel regions disposed in a first well region 141 (pwell region for a NMOS). The first well region 141 is disposed on alower doped substrate 142. For example, in one embodiment, the lowerdoped substrate 142 comprises a deep well region formed in a substrate,in other embodiments the lower doped substrate 142 comprises the dopingof the substrate before fabrication, for example, a p type doped waferin one embodiment. The first well region 141 is formed within the lowerdoped substrate 142. The first well region 141 comprises a same type ofdoping, in one embodiment.

A first ring 120 comprising a heavily doped region (p⁺ doping for aNMOS) is disposed on the first well region 141 as illustrated in FIG. 10a. A ring shaped second well region 131 is disposed under the secondring 130 (see FIG. 10 a). The second well region 131 is disposedadjacent the first well region 141 and separates the first well region141 into a first portion and a second portion. The second well region131 comprises an n⁻ doping for a NMOS transistor. A second ring 130 isdisposed in the second well region 131. A pickup region 140 comprising aheavily doped region (p⁺ doping for a NMOS) is disposed on the firstwell region 141 as illustrated in FIG. 10 a. The second ring 130 is thusdisposed between the first ring 120 and the pickup region 140. Thesecond ring 130 shields the first ring 120 from the active part of thesubstrate (e.g., the source/substrate junction of the MOS transistor100). Thus, the second ring 130 enables the pickup region 140 to tapinto the substrate potential more efficiently.

In the dual well process illustrated in FIG. 10 b, the substrate of theMOS transistor 100 is coupled to the body or substrate contact (firstring 120) through a lower doped region and hence through an effectivesubstrate resistance Rsub. This coupling of the substrate to thesubstrate potential line VSS is also depicted in various embodiments inabove Figures (e.g. FIGS. 4-8 illustrate this embodiment as effectivesubstrate resistance Rsub). In various embodiments, the first wellregion 141 is about 0.5 μm to about 5 μm deep, and about 2 μm in oneembodiment.

In contrast, in the triple well design (FIG. 10 c) a second well region143 is formed disposed within the lower doped substrate 142. The secondwell region 143 is deeper than the first well region 141 and comprisesan opposite type doping. Hence, the triple well design creates anisolated portion of the first well region 141. The isolated first wellregion 141 is shielded from the substrate contact (first ring 120) bysecond ring 130 laterally and vertically by the second well region 143.Hence, this results in optimal pickup of the potential under the activeMOS device.

FIG. 11 illustrates an alternate embodiment of the top view of MOStransistor described with respect to FIG. 10 a. Unlike FIG. 10 a, thepickup region 140 is placed centrally while the gate 150 is disposedbetween source regions 170 and drain regions 180 on either side of thedevice. As the pickup region 140 is placed centrally additionalshielding rings (example, second ring of FIG. 10 a) are not necessaryresulting in area savings. Although this layout is more efficient, adecrease in efficiency of pickup is likely. In some embodiments, a lowerefficiency of the potential pickup may be acceptable for the gains inarea.

In various embodiments, the ESD clamp device described above comprises adevice with a low capacitance unlike the RCMOS clamp illustrated forexample in FIG. 1 a. Hence, in various embodiments, the ESD clamp deviceis used for protection of local input/output (I/O) pads. In variousembodiments, the ABST clamp described above is applied to CMOS bulk, SOItechnologies with substrate or body contacts as well as bipolar and/ormixed signal technologies.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. For example,it will be readily understood by those skilled in the art that many ofthe features, functions, processes, and materials described herein maybe varied while remaining within the scope of the present invention.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

1. A semiconductor device comprising: a substrate; a first well regiondisposed in the substrate, the first well region comprising a firstdoping type; a first source/drain region disposed in the first wellregion, the first source/drain region being coupled to a first potentialnode; a second source/drain region disposed in the first well region,the first and the second source/drain regions comprising a second dopingtype, the second doping type being opposite to the first doping type,the second source/drain region being coupled to a second potential node;an OR logic block; a first gate electrode disposed above the substrate,the first gate electrode disposed between the first and the secondsource/drain regions, the first gate electrode coupled to an output ofthe OR logic block; and a pickup region disposed in the first wellregion, the pickup region comprising the first doping type, the pickupregion coupled to an input of the OR logic block.
 2. The device of claim1, wherein the substrate comprises a second well region formed in asubstrate, the first well region disposed in the second well region. 3.The device of claim 1, wherein the substrate comprises a dopedsubstrate.
 4. The device of claim 1, wherein the OR logic blockcomprises an OR gate.
 5. The device of claim 1, wherein the OR logicblock comprises a NOR gate coupled to a first inverting buffer, whereinthe first input of the OR logic block is a first input of the NOR gate,and wherein the second input of the OR logic block is a second input ofthe NOR gate, and wherein an output of the NOR gate is coupled to aninput of the first inverting buffer
 6. The device of claim 5, whereinthe NOR gate comprises a first PMOS transistor, a second PMOStransistor, a first NMOS transistor, a second NMOS transistor, wherein afirst source/drain of the first PMOS transistor is coupled to the firstpotential node, wherein a second source/drain of the first PMOStransistor is coupled to the first source/drain of the second PMOStransistor, and wherein a second source/drain of the second PMOStransistor is coupled to a first source/drain of the first NMOStransistor, wherein a second source/drain of the first NMOS transistoris coupled to the second potential node, wherein a first source/drain ofthe second NMOS transistor is coupled to the second source/drain of thePMOS transistor, wherein a second source/drain of the second NMOStransistor is coupled to the second potential node.
 7. The device ofclaim 6, wherein the first input of the NOR gate is coupled to a gate ofthe first PMOS and NMOS transistors, and wherein the second input of theNOR gate is coupled to the gate of the second PMOS and NMOS transistors.8. The device of claim 6, wherein the second source/drain of the secondPMOS transistor, and the first source/drain of the first and the secondNMOS transistors are coupled to the output node of the NOR gate.
 9. Thedevice of claim 5, wherein the first inverting buffer comprises a CMOSinverter, the input of the CMOS inverter being coupled to the output ofthe NOR gate, the output of the CMOS inverter being coupled to a gate ofthe first transistor.
 10. The device of claim 1 further comprising: asubstrate contact region disposed in the first well region, thesubstrate contact region being coupled to the second potential node. 11.The device of claim 6 further comprising a shielding region laterallyshielding the substrate contact region from the pickup region.
 12. Thedevice of claim 6 further comprising a second well region disposed underthe first well region, the second well region comprising the seconddoping type, the second well region vertically shielding the substratecontact region from the pickup region.
 13. The device of claim 6,wherein the substrate contact region is coupled to a first portion ofthe first well region adjacent the first source/drain region through asubstrate resistor, the substrate resistor comprising a second portionof the first well region and a portion of the substrate.
 14. The deviceof claim 1, wherein the first doping type comprises a p type doping, andthe second doping type comprises an n type doping.
 15. The device ofclaim 1, wherein the first potential node comprises a power supply node,and wherein the second potential node comprises a ground potential node.